Blowout prevention apparatus for subaqueous drilling

ABSTRACT

The apparatus includes a vessel carrying a derrick and composite main electric and hydraulic draw works, each being dead ended to the other. In normal operations, the main draw works is employed to raise and lower the drill string with the anchor line dead ended to the hydraulic draw works. When a &#39;&#39;&#39;&#39;kick&#39;&#39;&#39;&#39; or other abnormality in the well occurs and the blowout preventors are actuated, the drill string is lowered such that a tool joint rests on the blowout preventer rams. The hydraulic draw works is then actuated and provides a constant tension on the drill string whereby the rams support the drill string below the rams and the hydraulic draw works supports the drill string above the rams. The constant tension hydraulic draw works precludes relative motion between the drill string and the ram heads which motion would otherwise occur due to vessel motion.

United States Patent Lloyd 51 July 18,1972

[ BLOWOUT PREVENTION APPARATUS FOR SUBAQUEOUS DRILLING [72] inventor: Samuel H. Lloyd, Los Angeles, Calif.

[52] [1.8. CI ..l75/7, 175/27 [51] Int. Cl. ..E2lc 19/00 [58] Field ofSearch I 75/7, 27, 38, 5, 8, 9;

[56] References Cited UNITED STATES PATENTS 3,159,218 12/1964 Wilde ..l75/7 X 3,151,686 10/1964 Kammerer... .....l75/5 2,923,531 2/1960 Bauer et a]... .....l75/7 3,315,742 4/1967 Nicolson ..l75/7 X 3,457,991 7/1969 Sizer et al. ..l66/.5

3,474,858 10/1969 Gibson et al. ..l75/7 X Primary Examiner-Marvin A. Champion Assistant Examiner-Richard E. Favreau Attorney-be Blane & Shur [57] ABSTRACT The apparatus includes a vessel carrying a derrick and composite main electric and hydraulic draw works, each being dead ended to the other. In normal operations, the main draw works is employed to raise and lower the drill string with the anchor line dead ended to the hydraulic draw works. When a kick or other abnormality in the well occurs and the blowout preventors are actuated, the drill string is lowered such that a tool joint rests on the blowout preventer rams. The hydraulic draw works is then actuated and provides a constant tension on the drill string whereby the rams support the drill string below the rams and the hydraulic draw works supports the drill string above the rams. The constant tension hydraulic draw works precludes relative motion between the drill string and the ram heads which motion would otherwise occur due to vessel motion.

11 Claims, 7 Drawing figures Patented July 18, v 1972 3 Sheets-Sheet 1 FIG. 7

INVENTORS SAMUEL H. LLOYD ATTORNEYS Patented Jul 18, 1972 3,677,352

3 Sheets-Sheet 2 FIG. 2

FLUID SUPPLY,

CONTROLS a MOTOR MOTOR CLUTCH INVENTORS iiwutwg ATTORNEYS SAMUEL H. LLOYD 5 Sheets-Sheet 3 88\ RESERVOIR FIG 6 I00 96 Qt l I02 7 BRAKE 1]- {N G f L ACCUMULATOR $76 HEAT EXCHANGER RESERVOIR 78/ Z 74 INVENTORS J6 SAMUEL H. LLOYD ATTORNEYS BLOWOUT PREVENTION APPARATUS FOR SUBAQUEOUS DRILLING The present invention relates to apparatus for subaqueous drilling and particularly relates to apparatus for supporting a drill string under constant tension as to preclude movement of the drill string relative to the rams of a blowout preventer in the event of a kick or other abnormality in the well.

In conducting drilling operations, a drill bit not infrequently enters a formation containing gas or fluid at a higher pressure than the hydrostatic head of the drilling fluid resulting in an upward flow of gas and oil (sometimes water) within the well casing or annulus, i.e., a high pressure zone wherein the drilling mud is unable to hold the increased pressure. This upward flow of material is termed a kick" and, unless restrained, will result in a blowout, i.e., a relatively sudden and violent escape of the gas and oil (sometimes water) from the well. This phenomena occurs both in land and subaqueous drilling.

When drilling on land, it is conventional practice to employ blowout preventers to restrain this upward flow when and if it occurs. Generally, to preclude a blowout and to restrain a kick blowout preventer rams or bladders are provided which can be closed about the drill pipe to seal the opening. The sealing of the opening provides time in which heavier drilling fluid can be prepared and pumped inside the drill pipe. The heavier mud circulates into the annulus and back to the drilling rig through a choke. This is a rather lengthy although routine procedure with respect to drilling operations conducted on land i.e., where the derrick is in a fixed position. However, in subaqueous drilling the derrick is mounted on a vessel and drilling operations are often conducted while the vessel lies in a floating condition subject to heave motions, i.e., vertical oscillations and other motions induced by wave, wind and other forces acting on the vessel. The blowout preventer is, of course, located at the land-water interface. Accordingly if a kick occurs while drilling from a floating vessel subjected to heave motions, and the blowout preventer rams are closed about the drill pipe to seal the opening, the drill pipe will continue to vertically oscillate with the vessel relative to the fixed blowout preventer rams causing the ram seals to wear rather rapidly. This periodic vessel motion thus can wear the blowout preventer seals to the extent that the seals become ineffective to contain the high pressure upward flow with the result that a blowout occurs anyway before the well can be brought under control under standard procedures.

The present invention eliminates the hazards, when the blowout preventers are actuated, of wearing through the preventer seals due to the periodic motion of the drill pipe relative to the seals and provides a means for fully supporting the drill string in the hole without movement of the drill string relative to the blowout preventer rarns during the period that the preventer rams are closed and control of the well is being obtained. To this end, the present invention provides a conventional main draw works for supporting the drill string in the hole under normal operating conditions with the dead line being unconventionally anchored to the secondary hydraulically actuated draw works system which is employed in the event a kick or blowout occurs. The hydraulic draw works system includes a brake whereby the main draw works is free to operate in a conventional manner under normal conditions. Similarly, the main draw works system is also provided with a brake whereby it acts as the deadline or anchor for the hydraulic draw works system when the latter is actuated. The hydraulic draw works system includes a reversible variable displacement pump and a reversible fixed displacement motor coupled one to the other with the draw works reel being coupled to the motor or the motor being driven by the draw works reel depending on the motions of the vessel. Specifically, when the hydraulic system is actuated, a predetennined flow is provided by the pump to drive the fluid motor at a predetermined speed and torque to maintain a predetermined tension on the drill string. Should the tension on the drill string tend to increase, as for example, when the vessel heaves upwardly relative to the blowout preventers on the land-water interface, a control system is actuated to maintain a constant tension on the drill string whereby the latter is maintained in a constant position relative to the blowout preventers while the vessel moves upwardly relative to the fixed drill string. Conversely, when the vessel heaves downwardly, the control system is actuated to again provide the constant tension on the drill string with the result that the drill string is maintained in a constant position relative to the blowout preventers while the vessel moves downwardly relative to the fixed drill string. To this end, a pressure accumulator is provided in the fluid line between the pump and motor to provide quick response to load variations on the reel caused by vessel heave motions and prevents runaway if the load is to great. The present hydraulic circuit including the accumulator has the advantage of inherent feedback compensation for maintaining constant load tension.

In use, when drilling under normal conditions utilizing the main draw works system and a kick occurs, the main system is employed to lower the drill string such that a tool joint rests on the top of the blowout preventer rams which have been actuated to seal about the well opening and about the drill string pipe. Once the tool joint rests on the rams with the rams providing temporary support for the drill string, the hydraulic system is actuated as to maintain a constant tension on the drill string above the rams and the main draw works system is braked and deactivated. Particularly, the rams support the weight of the drill string below the rams whereas the hydraulic constant tensioning system supports the weight of the drill string from the vessel to a point just about rams. The extent of the drill string and hence its weight between the vessel and the rams is always a known factor. Accordingly, the hydraulic control system can be set to support the known weight of the drill string with load variations caused by vessel motion in heave and tending to raise and lower the drill string being compensated for by the constant tensioning hydraulic system whereby relative motion between the drill string and the blowout preventer rams is precluded.

A constant tension hydraulic draw works system cannot pratically be employed in lieu of the main draw works system as it would be to large and expensive for use on a vessel performing subaqueous drilling. For example, in average subaqueous drilling operations, about 600,000 pounds of drill string are employed and this would require a 4,000 horsepower hydraulic system to support such a load. To utilize a constant tension hydraulic system for this load, would be highly inefficient and impractical. By lowering the drill string such that a tool joint rests on the blowout preventer rams, a large proportion of the weight is supported by the rams leaving only the weight of the drill string between the vessel and the rams to be supported. This permits utilization of a conventional main draw works in combination with a smaller capacity constant tension hydraulic draw works wherein the weight of the drill string between the vessel and the well can be maintained under constant tension by the small capacity hydraulic draw works and thereby avoid motion between the drill string and the ram seals.

Accordingly, it is a primary object of the present invention to provide a constant load tensioning system for eliminating and/or substantially reducing wear on the blowout preventer rams employed in subaqueous drilling.

It is another object of the present invention to provide a combination main and hydraulic draw works system to optimize the time available for gaining control of a well after a kick has occurred.

It is still another object of the present invention to provide, in a blowout prevention system, a hydraulic draw works for maintaining a portion of the drill string under constant tension and hence immovable relative to the blowout preventer stack notwithstanding vessel motions in heave.

it is a further object of the present invention to provide a method for precluding wear of the seals on blowout preventer rams due to motion of the vessel and hence drill string relative to the well head in subaqueous drilling operations.

These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings wherein:

FIG. 1 is a side view, partially in elevation and partially in vertical section of a subaqueous drilling apparatus utilizing the principles of the present invention;

FIG. 2 is an enlarged vertical cross-sectional view with parts broken out for ease of illustration, schematically illustrating the arrangement of the hoist and draw works on the vessel;

FIG. 3 is a fragmentary vertical cross-sectional view of a blowout preventer stack at the land-water interface and illustrated in a normal drilling position;

FIG. 4 is a horizontal sectional view thereof taken generally about on line 4-4 in FIG. 3;

FIG. 5 is a fragmentary vertical cross-sectional view of the blowout preventers illustrating the preventer heads in sealing position about the drill pipe;

FIG. 6 is a schematic illustration of a hydraulic control system for the hydraulic draw works; and

FIG. 7 is an enlarged fragmentary side elevational view of another form of the present invention.

Referring now to FIG. 1, the subaqueous drilling apparatus of the present invention includes a floating platform or vessel P adapted to float in a body of water, the surface of which is indicated at W, and carrying conventional main, in the illustrated form, electrical draw works indicated at 10, and a derrick 12 for supporting a drill string 14. Particularly, derrick 12 includes a crown block 16 and a travelling block 18 having a dependent rotary book 20 carrying a swivel 22 coupled to the kelly 24, which, in turn, is coupled to the drill string 14, the latter being rotated by a rotary table, not shown, carried by platform P.

The bottom of the body of water, i.e., the land-water interface, is indicated at I. Among other conventional drilling equipment located at the well head H at the land-water interface, there is disposed a blowout preventer stack S having, in

this instance, a lower pair of blind rams 26 and an upper pair of pipe rams 28. The blowout preventer stack construction is per se conventional and forms no part of this invention. It is believed sufficient to note that rams 26 and 28 are fluid actuated with blind rams 26 being adapted to seal the central opening 30 through the preventer stack in the event of a kick or blowout. The pipe rams are similarly fluid actuated and have ram heads 32 and seals 33 shaped to conform to the drill pipe whereby the ram heads can be moved toward one another to seal opening 30 about the drill pipe. Also, there is illustrated a choke line 38 communicating with the annular opening for the usual purpose of allowing controlled release of well pressure. A kill line connection 40 (FIG. 3) is also pro vided as is conventional. Below the preventer stack S is the usual cylindrical casing 44 defining an annular space 46. Drill pipe 14 extends through the stack S within casing 44 and terminates at its lower end in a drill bit 48.

Draw works comprises, as is conventional, a series of hydraulic mechanical or electric power sources for driving hoisting reel 50 which carries hoisting line 52. Line 52 is sheaved about crown block 16 and travelling block 18 and is conventionally dead ended to the drilling platform. Under conventional drilling practice, should a kick occur, the blowout preventers are actuated to seal about the drill string to preclude egress of the high pressure fluid from annulus 46. One annulus 46 is closed off, conventional procedures can be adopted through the use of the choke and kill lines to preclude a blowout and stabilize the well. These procedures, however, are time consuming and require the blowout preventer seals about the drill pipe to be maintained at all times. This is not a problem in conventional land based rigs wherein the drilling platform and the blowout preventer stack are relatively stationary with respect to one another. However, in the instance of a vessel or platform conducting off-shore drilling operations wherein the vessel is subject to heave motions, principally wave induced heave motions, the vessel will oscillate vertically causing the drill string 14 to oscillate with the vessel. Under normal conditions, the drill string would thus reciprocate vertically with respect to the blowout preventer rams 32 and provide significant wear action on the ram head seals 33 which, of course, are fixed on the land-water interface I. This constant motion of the drill string relative to the seals may cause rapid deterioration and breakdown of seals 33 and rather than preventing the blowout, failure of the seals can, in fact cause a blowout. Thus, it has been found desireable and it is a particular feature of the present invention to eliminate the motion of the drill string relative to the blowout preventers to thereby preclude this relatively rapid wearing action on the seals.

To this end, the present invention provides an auxiliary drill string support system which applies a constant tension to the hoist lines whereby the travelling block is maintained at a substantially relatively fixed distance from the preventer ram heads to preclude movement between the drill string and the ram heads, notwithstanding heave motions of the vessel. To accomplish this, the hoist line 52, which is sheaved about the crown and travelling blocks, rather than being dead ended to the platform, as is conventional, is wrapped about a second reel 54 and dead-ended as by a suitable brake 56. Reel 54 is under the control of a fluid actuated constant tension apparatus to be described. In utilizing the present invention as will presently be described, it will be noted that hoist line 52 is dead-ended to auxiliary reel 54 during normal operations when draw works 10 are employed, and that the hoist line is in effect dead-ended to reel 50 as by a suitable brake, not shown, when the constant tension device is actuated. Further, it will be appreciated that the constant tensioning system and draw works 10 are operated alternatively rather than conjunctively, for reasons which will become apparent.

Referring now to FIG. 6, the constant tensioning system hereof includes a reversible variable displacement pump which is driven by a suitable prime mover, for example a diesel engine indicated at 62. Pump 60 lies in communication with a reversible fixed displacement fluid motor 64 by a pair of fluid lines 66 and 68. Motor 64 is coupled in driving relation to the reel 54 through suitable reduction gearing, not shown. It will be appreciated that for a given fluid flow input to motor 64, a substantially constant output speed and torque is provided reel 54. An accumulator 70 is provided in fluid line 68 for providing rapid response to variations in the load carried by reel 54. A charge pump circuit is provided including a charge pump 72, a reservoir 74, a filter 76, a heat exchanger 78, a bypass valve and a fluid line 82 for connecting charge pump 72, pump 60, heat exchanger 78, reservoir 74 and filter 76 in series, with fluid line 84 being connected in parallel on opposite sides of heat exchanger 78. The charge pump circuit provides a flow of fluid through the transmission for cooling purposes, sufficient fluid under pressure for control purposes, a positive pressure on the low pressure side of pump 60 and for internal leakage makeup. A case drain is provided at 86 and a reservoir in communication with fluid line 66 is provided at 88. A control valve 90 which can be operated by lever 92 is provided in a control circuit including fluid lines 94, 96, for regulating pump 60. Relief valves I00 and 102 are disposed in parallel on opposite side of motor 64.

The operation of the blowout prevention system of FIGS. l6 will now be described. Under normal operating conditions, main drawworks 10 operates in a conventional manner to raise and lower drill pipe 14 through the preventer stack S. In the event that a high pressure formation is encountered in the well resulting in an upsurge of high pressure fluid the blowout preventer rams are actuated to close about drill string 14. It will be appreciated that seals 33 engage about the drill pipe and against one another to close off annulus 46 to prevent egress of the upwardly surging high pressure fluid. as best illustrated in FIGS. 3 and 5, the drill string 14 is then slowly lowered by the main drill works 10 until the tool joint 104 rests on top of preventer ram heads 32. The main drawworks 10 .is then braked and locked to provide a dead-end anchor for fluid actuated reel 54, the operation of which will now be described. Before the main drawworks is deactivated and its brake set, the brake on the reel 54 isreleased. ln-

asmuch as the weight of the drill string between the blowout preventer stack S and vessel P is known, the operator, by actuation of control 92, controls pump 60 to provide a rate of fluid flow to motor 64 sufficient such that the load carried by reel 54, that is, the weight of the drill string between vessel P and blowout preventer stack S, can be maintained under constant tension. With the fluid actuated system in operation, it will be appreciated that the weight of the drill string between vessel P and blowout preventer stack S is maintained by the fluid circuit through reel 54 and that the weight of the drill string below the preventer stack S is carried by stack S. In other words, the weight of the drill string below stack S is not carried by reel 54. If the vessel and the preventer stack were maintained in fixed relative positions, i.e., in very calm waters, the fluid draw works system would thus support the weight of the drill pipe between platform P and stack S with the result that the drill string and seals 33 remain fixed relative to one another. However, since the vessel is subjected to heave motions, principally wave induced, the drill pipe 14 would ordinarily vertically reciprocate with the vessel. By actuating the fluid system illustrated in FIG. 6, a constant tension is applied to the drill pipe. For example, when the vessel heaves upwardly tending to carry the drill string upwardly therewith the apparent increase in load causes the fluid draw works system to rotate reel 54 in a direction to payout line while simultaneously maintaining constant tension on the drill pipe. Conversely, when the vessel heaves downwardly, the apparent reduction in weight on the line causes the fluid system to operate reel 54 in a direction to reel in line while simultaneously maintaining a constant tension on drill pipe 14. Consequently, drill pipe 14 is maintained in a fixed position relative to the blowout preventer stack S while the vessel heaves relative to the fixed drill pipe 14. This precludes wearing of the ram seals 33 by the periodic movement of the drill pipe relative to the seals and thereby precludes the occurrence of a blowout due to failure of the seals about the drill pipe.

Referring now to the embodiment hereof illustrated in FIG. 7, there is illustrated a derrick l2 similarly arranged as illustrated in FIG. 1 of the previously described embodiment and including a block 16, a travelling block 18, rotary hook 20, swivel 22 and a kelly, not shown. As previously described, hoisting line 52 is sheaved about crown block 16 and travelling block 18. Hoisting line 52 includes an end portion 100 which, rather than be dead-ended directly to the drilling platform is dead-ended to a constant tensioning fluid actuated cylinder 104 which, in turn, is secured to the platform. A mechanical brake 102 is interposed between cylinder 104 and the crown block 16 and, under normal drilling operations,

dead ends hoisting line 52 to the platform without imposing any loads on the constant tensioning cylinder. The constant tension fluid actuated cylinder 104 may be of any conventional type, for example, the cylinder shown in US. Pats. No. 3,208,728 or US. Pat. No. 3,l58,208.

In operation, hoist line 52 is dead-ended to mechanical brake 102 and drilling operations are conducted in the usual manner utilizing the main draw works 10. Should a kick occur, the drill string is lowered as previously described such that the tool joint rests on the blowout preventer ram heads. Draw works is then locked and brake 102 is released to impose the load of the drill string between the preventer heads and the platform on the constant tensioning cylinder 104. Should the vessel rise relative to the blowout preventer stack S the constant tensioning cylinder 104 operates to maintain a constant tension on pipe line 14 between the preventer stack and the platform and pays out line 100 over the crown block 16 whereby pipe 14 is lowered under constant tension relative to the vessel precluding movement of pipe 14 relative to the preventer head seals. When the vessel heaves downwardly, cylinder 104 operates to maintain a constant tension on pipe line 14 disposed between the preventer stack and the vessel by hauling in on line 100 over crown blocks 16 again preventing relative movement of pipeline l4 and the preventer head seals.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

What is claimed and desired to be secured by United States Letters Patent is:

l. A method for preventing a blowout in subaqueous drilling operations from a floating vessel subject to heave motions wherein a drill string depends from the vessel through a blowout preventer disposed adjacent the land-water interface, the blowout preventer having seals for sealing about the drill string comprising the steps of engaging the blowout preventer seals in sealing relation about the drill string, supporting the portion of the drill string below the blowout preventer from adjacent the land-water interface and applying a substantially constant tension to the portion of the drill string between the blowout preventer and the vessel to support said latter drill string portion from the vessel and to substantially preclude movement of the drill string relative to the blowout preventer seals.

2. The method according to claim 1 wherein the drill string has an enlarged diameter portion and including the step of lowering the drill string such that its enlarged diameter portion rests on said blowout preventer to support the lower portion of the drill string from the blowout preventer.

3. The method according to claim 1 wherein said support means includes drawworks carried by said vessel for supporting the entire length of drill string during normal drilling operations, and a constant tensioning device for supporting the portion of the drill string between the blowout preventer and the vessel and including the stop of actuating the constant tensioning device and deactivating the draw works when the lower portion of the drill string is supported adjacent the blowout preventer.

4. A blowout preventing system for subaqueous drilling from a vessel subject to heave motions comprising: a drill string, means on said vessel for effecting drilling operations including a drawworks for supporting said drill string from the vessel, a blowout preventer adjacent the land-water interface and including preventer heads for sealing about said drill string, said blowout preventer being adapted to support the portion of the drill string below said blowout preventer when said preventer heads seal about said drill string, and means carried by the vessel for supporting the portion of said drill string between the vessel and said blowout preventer including means for applying a constant tension to said latter drill string whereby wear on the preventer heads when sealed about said drill string is substantially precluded, and wherein said drawworks is electrically actuated and said constant tensioning means is fluid actuated.

5. A blowout preventing system for subaqueous drilling from a vessel subject to heave motions comprising: a drill string, means on said vessel for effecting drilling operations including a drawworks for supporting said drill string from the vessel, a blowout preventer adjacent the land-water interface and including preventer heads for sealing about said drill string, said blowout preventer being adapted to support the portion of the drill string below said blowout preventer when said preventer heads seal about said drill string, and means carried by the vessel for supporting the portion of said drill string between the vessel and said blowout preventer including means for applying a constant tension to said latter drill string whereby wear on the preventer heads when sealed about said drill string is substantially precluded, said constant tensioning means including a reel and a line coupled to said reel and said drill string for raising and lowering said drill string relative to the vessel, said constant tension means being responsive to vessel heave motion induced variations in the load of the drill string portion between the vessel and said preventer heads from a predetermined load on said reel to operate said reel in a direction to obtain said predetermined load.

6. A blowout preventing system for subaqueous drilling from a vessel subject to heave motions comprising: a drill string, means on said vessel for effecting drilling operations including a drawworks for supporting said drill string from the vessel, a blowout preventer adjacent the land-water interface and including preventer heads for sealing about said drill string, said blowout preventer being adapted to support the portion of the drill string below said blowout preventer when said preventer heads seal about said drill string, and means carried by the vessel for supporting the portion of said drill string between the vessel and said blowout preventer including means for applying a constant tension to said latter drill string whereby wear on the preventer heads when sealed about said drill string is substantially precluded.

7. Apparatus according to claim 1 wherein said drill string includes an enlarged diameter portion, said enlarged portion resting on said blowout preventer heads when the latter are sealed about said drill, string.

8. Apparatus according to claim 6 wherein said drawworks includes a reel and a line coupled to said reel and said drill string for raising and lowering said drill string relative to the vessel, said constant tension means including a constant tensioning fluid actuated cylinder coupled to said line, a releasable brake interposed in said line between said constant tensioning fluid cylinder and said drill string for deadending said line when said drawworks is operable to raise and lower said drill string, and means coupling said cylinder to the vessel.

9. Apparatus according to claim 6 wherein said drawworks includes a reel and a line coupled to said reel and said drill string for raising and lowering said drill string relative to the vessel, said constant tension means including a second reel coupled to said line and being responsive to vessel heave motion induced variations in-the load of the drill string portion between the vessel and said preventer heads from a predetermined load on said second reel to operate said second reel in a direction to obtain said predetermined load.

10. Apparatus according to claim 9 including a brake for said second reel for deadending said line when said first men tioned reel is operable to raise and lower said drill string.

ll. Apparatus according to claim 9 including a brake for said first mentioned reel for deadening said line when said second reel is operable.

. UNE'EED sir/urns-mfnazwr 0mm (Tr-3mm; Mr or (30mm Patent N 3,677,352 Dated July 18, 1972 Inventor(s) Samuel H. Lloyd It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In Column 3, line 63, "One" should read --Once--.

In Column 4, line 7, "desireable" should read -desirable--f In Column 5, line 54, "U.S. Pat. No. 3,158,208" should read 3 ,l58,208-- In Column 6, line 36, Claim 3, "stop" should read -step--.

Signed and sealed this 9th day of January 1973.,

(SEAL) Attest:

ROBERT GOTTSCHALK JR. EDWARD M'FLETCHER Commissioner of Patents Attesting Officer FORM PO-105O (IO-69) 

1. A method for preventing a blowout in subaqueous drilling oPerations from a floating vessel subject to heave motions wherein a drill string depends from the vessel through a blowout preventer disposed adjacent the land-water interface, the blowout preventer having seals for sealing about the drill string comprising the steps of engaging the blowout preventer seals in sealing relation about the drill string, supporting the portion of the drill string below the blowout preventer from adjacent the land-water interface and applying a substantially constant tension to the portion of the drill string between the blowout preventer and the vessel to support said latter drill string portion from the vessel and to substantially preclude movement of the drill string relative to the blowout preventer seals.
 2. The method according to claim 1 wherein the drill string has an enlarged diameter portion and including the step of lowering the drill string such that its enlarged diameter portion rests on said blowout preventer to support the lower portion of the drill string from the blowout preventer.
 3. The method according to claim 1 wherein said support means includes drawworks carried by said vessel for supporting the entire length of drill string during normal drilling operations, and a constant tensioning device for supporting the portion of the drill string between the blowout preventer and the vessel and including the stop of actuating the constant tensioning device and deactivating the draw works when the lower portion of the drill string is supported adjacent the blowout preventer.
 4. A blowout preventing system for subaqueous drilling from a vessel subject to heave motions comprising: a drill string, means on said vessel for effecting drilling operations including a drawworks for supporting said drill string from the vessel, a blowout preventer adjacent the land-water interface and including preventer heads for sealing about said drill string, said blowout preventer being adapted to support the portion of the drill string below said blowout preventer when said preventer heads seal about said drill string, and means carried by the vessel for supporting the portion of said drill string between the vessel and said blowout preventer including means for applying a constant tension to said latter drill string whereby wear on the preventer heads when sealed about said drill string is substantially precluded, and wherein said drawworks is electrically actuated and said constant tensioning means is fluid actuated.
 5. A blowout preventing system for subaqueous drilling from a vessel subject to heave motions comprising: a drill string, means on said vessel for effecting drilling operations including a drawworks for supporting said drill string from the vessel, a blowout preventer adjacent the land-water interface and including preventer heads for sealing about said drill string, said blowout preventer being adapted to support the portion of the drill string below said blowout preventer when said preventer heads seal about said drill string, and means carried by the vessel for supporting the portion of said drill string between the vessel and said blowout preventer including means for applying a constant tension to said latter drill string whereby wear on the preventer heads when sealed about said drill string is substantially precluded, said constant tensioning means including a reel and a line coupled to said reel and said drill string for raising and lowering said drill string relative to the vessel, said constant tension means being responsive to vessel heave motion induced variations in the load of the drill string portion between the vessel and said preventer heads from a predetermined load on said reel to operate said reel in a direction to obtain said predetermined load.
 6. A blowout preventing system for subaqueous drilling from a vessel subject to heave motions comprising: a drill string, means on said vessel for effecting drilling operations including a drawworks for supporting said drill string from the vessel, a blowout preventer adjacEnt the land-water interface and including preventer heads for sealing about said drill string, said blowout preventer being adapted to support the portion of the drill string below said blowout preventer when said preventer heads seal about said drill string, and means carried by the vessel for supporting the portion of said drill string between the vessel and said blowout preventer including means for applying a constant tension to said latter drill string whereby wear on the preventer heads when sealed about said drill string is substantially precluded.
 7. Apparatus according to claim 1 wherein said drill string includes an enlarged diameter portion, said enlarged portion resting on said blowout preventer heads when the latter are sealed about said drill string.
 8. Apparatus according to claim 6 wherein said drawworks includes a reel and a line coupled to said reel and said drill string for raising and lowering said drill string relative to the vessel, said constant tension means including a constant tensioning fluid actuated cylinder coupled to said line, a releasable brake interposed in said line between said constant tensioning fluid cylinder and said drill string for deadending said line when said drawworks is operable to raise and lower said drill string, and means coupling said cylinder to the vessel.
 9. Apparatus according to claim 6 wherein said drawworks includes a reel and a line coupled to said reel and said drill string for raising and lowering said drill string relative to the vessel, said constant tension means including a second reel coupled to said line and being responsive to vessel heave motion induced variations in the load of the drill string portion between the vessel and said preventer heads from a predetermined load on said second reel to operate said second reel in a direction to obtain said predetermined load.
 10. Apparatus according to claim 9 including a brake for said second reel for deadending said line when said first mentioned reel is operable to raise and lower said drill string.
 11. Apparatus according to claim 9 including a brake for said first mentioned reel for deadening said line when said second reel is operable. 